Ygam, did you see the recent discussion about Ken Rockwell's web site? See photo.stackexchange.com/questions/10980/…. In short, while Ken Rockwell has some interesting and helpful articles, he also compares himself to spoof site The Onion. My suggestion is to edit the question to remove those references (so the answer can be more helpfully focused on the lenses themselves).
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mattdmApr 25 '11 at 0:26

5 Answers
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The biggest reason for difference in the two lenses is aperture. The 80-200mm is a constant f/2.8 throughout the focal range and the 18-200mm varies from f/3.5 to f/5.6, so substantially slower, especially at the far end. All this really means is that the 80-200 can let in more light at the same focal length over the other.

Also, generally, zooms with constant apertures are higher grade lenses. I hesitate to make this statement a truism, but it pretty much is. Consumer grade lenses are often massive zoom ranges with variable aperture whereas more professional grade variants are smaller zoom ranges and constant apertures. The professional variants tend to be fast, sharper, and better built. There are exceptions, but this is generally the case regardless of brand.

So, that is why you might make the choice of the 80-200mm lens. I made such a choice with a Pentax variant not so long ago...

Edit

To answer your other question, I would probably recommend the super zoom (18-200) for a newcomer if you want a single, general purpose, lens. I would expect the 80-200 to be optically superior, but also more expensive, and it would probably mean a second, equally expensive, lens to fill in the range. Worth it to some of us, but not for everyone.

Keep in mind too that, even though the 80-200 may be a "better" lens, it may not be a better lens for you. You may want to trade off the better aperture, build quality, image quality for smaller, lighter, cheaper, and wider zoom.
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Craig WalkerApr 25 '11 at 0:50

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@Craig Walker - This is true, I know people who have done that, myself included. However, if you do get hooked on this photography thing, you'll regret that decision... :)
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John Cavan♦Apr 25 '11 at 0:55

Main reason for staying away from the 80-200mm 2.8 is the size. It's not practical for anyone except super enthusiast or professionals. The size alone makes it unjustifiable for any beginner.
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grmApr 26 '11 at 20:42

Nobody is saying that you must choose an 80-200 over an 18-200. An 80-200 (f/2.8) has some severe drawbacks compared to an 18-200, price, size and weight being among them besides the obviously limited zoom range. On the other hand, an 80-200 is far better behaved optically; it will tend to focus faster and more accurately (on a given camera body, and assuming similar focus motor technology as the superzoom), it can let in four times more light at 200 mm (f/2.8 versus f/5.6) which can make all the difference. And if you are attacked by a rabid bull, you can easily beat it to death with the lens and still snap a photo of the carcass afterwards.

An 18-200ish superzoom tries to do all things and does nothing particularly well; an 80-200 f/2.8 does only one thing and does it supremely well. It is one of the bread and butter lenses for journalists. I have one myself. But, I must add, I have carried it around all day long as a tourist and that is no fun at all. Add a normal zoom to supplement the missing wide-angle of this lens and it is even less fun to lug it all around. Which is, I suspect, why superzooms were invented in the first place.

You should keep in mind that for a superzoom to work well, you have to have rather good light. As in bright daylight, preferably. An f/5.6 aperture is quite limiting both in terms of exposure and in terms of autofocus.

As a really, really broad generalization, it's easier to make a high-quality zoom lens with a smaller zoom range, rather than a larger one. Although it's really tempting to look for one zoom that'll cover your entire range of shooting (Tamron's 18-270 comes to mind), these lenses tend to be fairly ill-behaved over portions of their range (at least), and suffer from softness and complex distortions that aren't easily fixed in post-processing.

The two lenses you indicate are really fairly different; the 80-200 is a fast lens (f/2.8), while the 18-200 is quite a bit slower. You'd be giving up a lot of optical performance to get that extra range, which is why most people end up covering this range with two lenses, more or less.

Usually lenses with a large focal ranges end up having poorer quality than lenses for each "normal" focal range (tele, wide, regular zoom). You get more versatility with lower image quality, it's a case of "Jack of all trades, master of none".